Kinetics and mechanism of the oxidation of some heterocyclic secondary alcohols by N-bromoacetamide in acid medium

Abstract

The kinetics of oxidation of 12 epimeric pairs of 1-hetera-2,6-diphenylcyclohexan-4-ols by N-bromoacetamide in the presence of perchloric acid in aqueous acid have been investigated. The oxidation is first-order in both oxidant and substrate and of minus-one-order in acetamide (an intermediate in the reaction) at constant acid concentration. The order with respect to H₃O⁺ is observed to be unity at constant ionic strength in perchloric acid. Based on the observed deuterium kinetic isotope effect in the case of the epimeric pairs, 2,6-diphenyl-3, N-dimethylpiperidin-4-ols and 2,6-diphenyl-3- ethyl-N-methylpiperidin-4-ols, a mechanism involving the participation of an O–H bond in the rate-limiting step is proposed. The reactivities of various 1-heteracyclohexan-4-ols towards oxidation have been rationalised on the basis of conformational differences. The effect of solvent polarity on the rate has been studied. Activation parameters have also been evaluated.